This invention relates to a process and apparatus for the surface hardening of workpieces made from refractory metals or metal alloys containing refractory metals and particularly such a process and apparatus for workpieces made from such refractory metal or alloys and utilized as bearings, valves, or similar products which are subjected to wear or abrasion.
A group of metals known as refractory metals consisting of zirconium, tantalum, titanium, hafnium, niobium and some others, have a common characteristic in that oxygen and nitrogen can penetrate and/or react with the surface of the metal to form a hardened case a few thousandths of an inch thick, and simultaneously build barrier compounds of oxides or nitrides on the surface, which prevent or limit further penetration. The characteristic is also observed with alloys of metals wherein at least the major metal portion is a refractory metal. The oxides and nitrides which form on the surface are extremely hard and wear resistant, but are very thin. The deeper or thicker cases which form beneath the surface are sometimes less hard, but have much greater depth, are less brittle, as they are made up of alloys of the base metal with oxygen or nitrogen rather than oxides or nitrides thereof. Oxides which form on the surface of these metals are known as ceramics and are very dense, hard and abrasion resistant. Nitrides which form are also separate compounds and are extremely hard and abrasion resistant. By appropriate combinations of temperature, atmospheres and other hardening techniques, it is possible to form combinations of hard surface compounds and alloyed sub surface cases which have very desirable properties.
Zirconium has long been recognized as a highly corrosion resistant material for severe applications. However, zirconium is relatively soft, about 65 Rockwell B, and is easily marred or damaged. It has not heretofore been suitable for heavy dynamic contact such as metal seals and wear parts. A number of previous studies indicated that zirconium could be case hardened by oxidizing the surface at temperatures about 1000.degree. F. With careful control in a laboratory environment, a ceramic zirconium oxide surface nearly one (1) mil thick can be formed. Further, zirconium metal beneath the oxide surface can be hardened by alloying with oxygen.
However, there is a critical time and temperature relationship for hardening zirconium by oxidizing in order to obtain the desired hard and dense film. If heated for too long a period of time at a relatively high temperature, the zirconium alloy workpiece may be seriously damaged. Under isothermal heating, the rate of hardening as measured by oxygen pickup will decrease with time. During this period of decreasing rate of oxygen pickup, a dense, tough, tightly adhering, blue-black case will form without any effect on the surface finish, and without any significant distortion of the part. However, continued heating will result in a fairly sudden increase in oxidation rate, and a case which is less abrasion resistant, brittle, and rough-surfaced will form. In addition, significant dimensional changes may take place.
The borderline between the conditions which form desirable cases and those which are over-oxidized is critical, and the results of excess oxidation are severe, so production practice has been very conservative using relatively low temperatures and accepting cases much less than optimum. Such cases are suitable for most uses and do provide a degree of resistance against marring, but they are substantially less than theoretically possible, and are not suitable for heavy sliding contact or abrasive wear for prolonged periods of time.
As indicated, zirconium has superior corrosion resistance properties and is utilized extensively in the chemical processing industry particularly where high operating temperatures and/or pressures are involved in an aqueous media. However, zirconium has a relatively low resistance to abrasion and in order to increase its resistance to abrasion and resulting wear, it is necessary to harden the wear surfaces. Heretofore, such as shown in U.S. Pat. No. 4,671,824 dated Jun. 9, 1987, a process is disclosed for a hardened wear surface from providing a zirconium alloy surface by treating the zirconium alloy in a heated molten salt bath containing small amounts of sodium carbonate which is an oxygen bearing compound. The thickness of the blue-black coating formed by this process by oxidation of the zirconium alloy was not specified but was defined as a relatively thin coating.
A fluidizing bed for forming a hardened layer on a workpiece has been utilized heretofore for certain workpieces such as illustrated in U.S. Pat. Nos. 4,141,759; 4,547,228; and 4,923,400 for example. An inert gas and various metal treatment processes such as nitriding or oxidizing have also been utilized with a fluidized bed as shown in these references. However, the use of a fluidized bed for refractory metal workpieces, which naturally form barrier compounds to the infusion of reactive gases and particularly a fluidized bed of oxide materials having an affinity for the reactive gas, or metal oxide wherein the metal has an affinity for oxygen, at least as great as the refractory workpieces has not been shown by the prior art.
The hardening of reactive metals has been accomplished in a number of ways heretofore. However, such hardening operations have been characterized by the formation of a hard chemical compound of the workpiece metal and the reactive gas on the outer surface, without the benefit of deeper harder surfaces as the chemical reaction on the outer surface prevents or limits diffusion of the reactive ions for creating the deeper alloy case.